Electronic components such as resistors, capacitors, and inductors are typically tested in a high speed handler. Testing the components includes two steps: (1) performing lead verification to determine whether the contact resistance between the leads of the component and test probes is within an acceptable range, and (2) performing component tests to measure certain parameters such as capacitance, dissipation factor, inductance, and resistance. Contact resistance is due to, for example, oxidization on the component leads. Kelvin 4-terminal testers are commonly used for performing component testing but eliminate the lead and contact impedances from the measurement only to a particular magnitude.
FIG. 1 shows a prior art circuit 10 which employs two 2-terminal testers for lead verification. A device under test ("DUT") component 14 includes bottom lead 16 and top lead 18. A low impedance drive (or excitation) probe 22 and a low impedance sense probe 24 are connected to opposite sides of lead 16. Probes 16 and 22 are also connected to a resistance meter 30. The total resistance in the lower loop of circuit 10 includes the resistance in lead 16, which is typically negligible, the resistance of the contact between drive probe 22 and lead 16, and the resistance of the contact between sense probe 24 and lead 16. The total resistance also includes the resistance in probes 22 and 24, and the resistance in the resistance meter 30. The total resistance in the entire lower loop may be determined with resistance meter 30. An identical measurement is performed to the upper circuit.
The resistance in probes 22 and 24 and resistance meter 30 can be known. The resistance in lead 16 is typically very small or negligible. By subtracting the known resistances from the total resistance, the resistance of the two contacts can be determined. If the resistance of the two contacts is above a maximum, for example, 100 milliohms, then the component test will not be reliable.
FIG. 2 shows a prior art circuit 40 that employs a Kelvin 4-terminal tester for testing components for parameters such as resistance and capacitance. Low impedance drive probe 52 and very high impedance sense probe 54 are connected to opposite sides of the lead 48. A current is sent through drive probe 52 from component test meter 42. Virtually all of the current from drive probe 52 goes through component 44 to lower drive probe 60. The current returns to component test meter 42 through drive probe 60, which is positioned on the opposite side of lead 46 from high impedance sense probe 64. Sense probes 54 and 64 sense the voltage across component 44.
Heretofore, lead verification has been performed as a separate test from component parameter tests. Performing the tests separately causes two problems. First, although the leads are verified before and/or after the component test, the contact resistance between the probes and the lead may change after the contact resistance is measured but before the completion of the testing of the component, leading to unreliable component test results. Second, the serial nature of the method increases the total time used to test the component. There is, therefore, a need to increase both the accuracy and the speed of lead verification and component testing.